Fluid cylinder

ABSTRACT

A pressure fluid cylinder having a tubular casing, a piston engaged reciprocably within the casing, two pressure chambers within the casing one on each side of the piston, a piston rod connected to the piston, an annular brake band with radial selfreturning force interposed between the outer wall of the piston and the inner wall of the casing, means operative only when in its neutral position to outwardly expand the brake band to frictionally engage the inner wall of the casing and to release the brake band when out of its neutral position, resilient means for normally conditioning the operating means to its neutral position, and pressure receiving means for moving the operating means out of its neutral position upon supply of fluid pressure into either of the two pressure chambers.

United States Patent 1191 Sugimoto 11 3,733,971 14 1 May 22, 1973 [73]Assignee: Toyooki Kogyo Kabushiki Kaisha,

Okazaki-shi, Aichi-ken, Japan [22] Filed: Dec. 22, 1971 [21] Appl. No.:210,708

[30} Foreign Application Priority Data Dec. 28, 1970 Japan ..45/l279913,665,812 5/1972 l-lashimoto et al ..92/27 X Primary Examiner-Martin P.Schwadron Assistant Examiner-Abe l-lershkovitz Attorneyl-lyman Berman etal.

[57] ABSTRACT A pressure fluid cylinder having a tubular casing, apiston engaged reciprocably within the casing, two pressure chamberswithin the casing one on each side of the piston, a piston rod connectedto the piston, an annular brake band with radial self-returning forceinterposed between the outer wall of the piston and the inner wall ofthe casing, means operative only when in its neutral position tooutwardly expand the brake band to frictionally engage the inner wall ofthe casing and to release the brake band when out of its neutralposition, resilient means for normally conditioning the operating meansto its neutral position, and pressure receiving means for moving theoperating means out of its neutral'position upon supply of fluidpressure into either of the two pressure chambers.

12 Claims, 6 Drawing Figures PATEmEW 3,733,971

SHEET 1 OF 2 INVENTOR IMA SASH/ 506/ MO 70,

I BY 2 FLUID CYLINDER BACKGROUND OF THE INVENTION This invention relatesto a pressure fluid cylinder, and more particularly to an apparatus forlocking the piston and piston rod of the cylinder at a desired position.

In the prior art, pressure fluid cylinders have generally been used inconjunction with selector valve means to regulate their operation. Suchselector valve means usually control the supply and discharge ofoperation fluid for the cylinder, thus regulating the positioning of thepiston within the cylinder. Therefore, it has often occurred that theposition of the piston is changed due to leakage of the fluid from theselector valve means and/or in accordance with the variation of the loadapplied to the piston rod. This is especially true with pneumaticallyoperated fluid cylinders. Thus, conventional cylinders are unable tooperate with controlled accurate positioning of their pistons andparticularly, where multiple positioning is involved, the problem isintensified. Furthermore, in the event that an external load is appliedto the piston rod, no desired positioning of the piston can be secured.

SUMMARY OF THE INVENTION A prime object of the present invention is,therefore, to provide a pressure fluid cylinder having a piston and apiston rod which can be integrally conditioned by proper selector valvemeans to a desired position when the same fluid pressure exists in bothof the chambers defined in the cylinder casing at the opposite sides ofthe piston, the piston and the piston rod then being locked firmly atthe desired position.

Another object of the present invention is to provide a pressure fluidcylinder, having the abovementioned characteristics, wherein the lockingof the piston can be released smoothly at the start of movement of thepiston by the use of proper selector valve means.

It is another object of the invention to provide a pressure fluidcylinder, having the above-mentioned characteristics, wherein thelocking of the piston and the piston rod can bemaintained for a desiredlengthy period of time regardless of any leakage of operation fluidwhich may occur.

It is still another object of the present invention to provide apressure fluid cylinder, having the abovementioned characteristics,wherein the secure locking of the piston and the piston rod can beattained regardless of application of an external force onto the pistonrod.

It is a further object of the present invention to provide a pressurefluid cylinder, having the abovementioned characteristics, wherein brakemeans is provided for locking of the piston and the piston rod, saidbrake means being capable off carrying out its frictional engagement anddisengagement very smoothly.

A still further object of the present invention is to provide a pressurefluid cylinder, having the abovementioned characteristics, wherein thestructure is very compact, and yet a very effective braking force isobtained.

According to the present invention briefly summarized, there is provideda pressure fluid cylinder of the type having a tubular casing, a pistonengaged reciprocably within the casing, two pressure chambers beingformed by the piston and the casing at the opposite sides of the piston,respectively, and a piston rod connected to the piston, wherein theimprovement comprises a locking device for the piston in the form ofannular brake means with radial self-returning force interposed betweenthe outer wall of the piston and the inner wall of the casing, operatingmeans for outwardly expanding the brake means to frictionally engage theinner wall of the casing while said operating means is in its neutralposition, and for releasing the brake means while said operating meansis out of its neutral position, resilient means for normallyconditioning the operating means to its neutral position, and pressurereceiving means for moving the operating means out of its neutralposition upon supply of fluid pressure into either of the two pressurechambers, the pressure receiving means including an annular pressurereceiver axially reciprocable within the piston to form a subpressurechamber and a fluid passageway to connect one of the two pressurechambers with the subpressure chamber.

BRIEF DESCRIPTION OF THE DRAWINGS The above-mentioned and other objectsand features of the present invention will become more apparent from thefollowing description, especially when read in conjunction with theaccompanying drawings, in which:

FIG. 1 is an axial cross-sectional view of a pressure fluid cylinderaccording to the present invention;

FIG. 2 is a cross-sectional view taken along the line II-II of FIG. 1;

FIG. 3 is a partial cross-sectional view taken along line III-III ofFIG. 2;

FIG. 4 is an axial cross-sectional view, similar to FIG. 1, illustratinga pressure fluid cylinder according to another embodiment of the presentinvention;

FIG. 5 is a view of elevational cross-section taken along line V-V ofFIG. 4; and

FIG. 6 is a fragmentary cross-sectional view taken along line VI-Vl ofFIG. 5.

In the drawings like reference numerals designate like, or correspondingparts throughout all the Figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings,particularly to FIGS. 1 to 3, there is illustrated a preferredembodiment of a pressure fluid cylinder constructed in accordance withthe present invention, comprising a tubular cylinder casing l in whichis reciprocably engaged a piston means 4 subdividing the casing 1 intotwo cylinder chambers 23a and 23b. A central through bore 5 is axiallydrilled within the piston 4. The piston means 4 further comprises alarger diametered recess 8 bored at the left end portion thereof.Fixedly engaged within the through bore 5 is a smaller diametered rightend portion 3 of a piston rod 2. At the right end of the portion 3 ofthe rod 2 is a threaded portion 6 to which a lock nut 7 is applied tofasten the rod 2 firmly onto the pis- I ton 4, the larger diameteredportion of rod 2 forming a shoulder held securely against the bottomwall of recess 8.

Within the recess 8 of the piston 4 is provided an annular pressurereceiver plate 11 in axially slidable contact with the innercircumferential wall of the recess 8 and with the outer circumferentialwall of the piston rod 2. An annular stop member 27 is also fixed withinthe recess 8 so as to restrict leftward movement of the plate 11.

A pair of plungers 12,12 are reciprocably engaged within a pair ofthrough bores 9,9 provided within the piston 4 parallel to the bore 5.The left end of each of the plungers 12,12 is firmly fixed to thepressure re ceiver plate 11 by a stop ring 13. Each of the plungers12,12 has a pair of arcuately cross-sectional grooves 14a and 14bannularly formed about the outer circumferential wall thereof. The twogrooves 14a and 14111 are separated by a land 14c.

Referring to FIGS. 2 and 3, a second pair of plungers 19,19 arereciprocably engaged within a pair of through bores 10,10 drilled withinthe piston parallel to the bore 5. The left end of each of the plungers19,19 is firmly fixed to the pressure receiver plate 11 by a stop ring20, while the right end of each plunger has a flange 21. A coiled spring22a is disposed between the pressure receiver plate 11 and the base of acounterbore a provided at the left end of each of the bores 10,10.Another coiled spring 22b is interposed between the flange 21 of each ofthe plungers 19,19 and the base of a counterbore 10b drilled at theright end of each of the bores 10,10. Thus, the plungers 19,19 arenormally conditioned to the neutral position thereof by outwardlybiasing force of the coiled springs 22a and 22b. The movement of theplungers 12,12 corresponds with that of the associated plungers 19,19,all four plungers being fixed to plate 11.

An annular groove 16 is formed about the outer circumference of thepiston 4. Within the groove 16 are received a pair of annular brakebands 15,15 made of a resilient material such as synthetic resin, or thelike. A part of each of the brake bands 15,15 is cut off and removed toleave, at positions diametrically opposite each other, the radialcut-off faces a,15a and l5b,15b tapered outwardly toward each other, asshown in FIG. 2.

A pair of balls 17,17, each ball cooperating with one of the plungers12,12, are placed within radial bores 18 in the piston 4 so as toposition a portion of each ball between the cut-off faces 15a and 15b ofone brake band. When the plunger 12 is in its neutral position, theassociated ball 17 is thrust outwardly between the cutoff faces 15a and15b by the land 14c of the plunger 12 so that the brake band 15 isexpanded to frictionally engage the inner circumferential wall of thecasing 1. When the plunger 12 is displaced, the ball 17 is retractedinto one of the annular grooves 14a or 14b by return resilient force ofthe brake band 15.

A chamber 25 formed between the pressure receiver plate 11 and the rightend face of the recess 8 is connected with the cylinder chamber 23bthrough a passageway 24 axially disposed in the piston rod 2.

The novel operation of the fluid cylinder, according to this embodiment,is described in detail hereinafter. For the sake of convenience, thefunctional operation is explained using pneumatic fluid as the pressuremedium.

FIGS. 1 to 3 show the state of the device wherein the pressure in bothof the cylinder chambers 23a and 23b is equal to atmospheric pressure.Under this condition, the plungers 12,12 are at their neutral positions,as clearly shown in FIG. 1, because of the biasing force of the coiledsprings 22a,22a and 22b,22b exerted on the plungers 19,19 and thepressure receiver plate 11.

The balls 17,17 are outwardly urged in radial bores 18,15 by the lands140,14c of the plungers 12,12 to consequently enter into the spacesformed between the tapered cut-off faces 15a,15a and l5b,15b of thebrake bands 15,15. Thus, the bands 15,15 are strongly expanded intofrictional engagement with the inner wall of the casing 1, and thepiston means 4 is thus firmly locked against the casing 1. Under thisstate, even if a load is given, added or removed to, or from the pistonrod 2, the piston means 4 is kept firmly in its locked position withinthe tubular cylinder casing 1. In this manner, the piston rod 2 togetherwith the piston means 4 can be stably held at the predetermined, lockedposition without moving for a desired period of time.

When pneumatic pressure is supplied into the cylinder chamber 23a to acton the pressure receiver plate 11, while the pressure in the chamber 23bremains equal to atmospheric pressure, the plate 11, plungers 12,12 andthe assistant plungers 19,19 will be pushed to the right against thebiasing force of the spring means 22a,22a. At this moment, the fluid inthe chamber 25 is exhausted into the chamber 23b through the passageway24. The resilient forces of the brake bands 15,15 thrust the balls 17,17inwardly into the arc grooves 14:11 M when the plungers 12,12 move tothe right. Thus, because of the resilient forces of the brake bands 115,15 their cut-off faces 15a,15a and l5b,15b return to their closed, ornon-braking positions, frictionally disengaging the brake bands 15,15from the inner circumferential wall of the cylinder casing 1, andallowing the piston 4 and the piston rod 2 to move smoothly to theright.

When atmospheric pressure is applied to both chambers 23a and 23b therightward movement of the piston rod 2 and the piston 4 will stop, andthe pressure receiver plate 11 and the plungers 12,12 and 19,19 togetherare instantaneously returned to their original neutral positions, asshown in FIG. 1, by the biasing force of the spring means 22a,22a. This,again, causes balls 17,17 to be positioned on the lands 140,140 of theplungers 12,12 and to expand the brake bands 15,15 which again engagefrictionally with the inner circumferential wall of the cylindercasing 1. Thus, the piston rod 2 as well as the piston means 4 can bestably held at desired positions without moving for any desired periodof time. i

When pneumatic pressure is delivered into the cylinder chamber 23b whilethe pressure in the chamber 23a is equal to atmospheric pressure, thehigher pneumatic pressure is lead into the chamber 25 through thepassageway 24 and pushes the pressure receiver plate 11 leftward againstthe biasing force of the springs 22b,22b. Thus, the plungers 12,12 and19,19 are urged leftward along with the plate 11. The balls 17,17 arethrust into the grooves 1412,1412 by the return force of the brake bands15,15, frictionally disengaging the brake bands from the innercircumferential wall of the casing 1. When both chambers 23a and 23b areconnected to atmospheric pressure, the biasing force of the springs22b,22b returns the plate 11 together with the plungers 12,12 and 19,19to their original positions. The balls 17,17 are thrust into the spacesbetween the band cut-off faces 1511,1517 to expand the bands andcomplete frictional engagement between the brake bands 15,15 and theinner wall of the casing 1.

As has been described in detail, so long as a source of pneumaticpressure is not connected to either one of the cylinder chambers 23a and23b, the balls 17,17 stay on the lands 140,140 of the plungers 12,12,expanding the brake bands 15,15 to engage frictionally with the innercircumferential wall of the tubular cylinder casing 1. Thus, due to thelocking effect of the brake bands 15,15 against the casing 1, the pistonrod 2 as well as the piston 4 can be stably held at the predeterminedposition without moving for a desired period of time, regardless of anyvariation of the load against the rod 2 and/or leakage of operationfluid.

Upon delivery of pneumatic pressure to either one of the cylinderchambers 23a and 23b, the locking of the brake bands 15,15 against thecasing 1 is instantaneously released so as to enable the piston rod 2and the piston 4 to move smoothly, respectively, to the right, or to theleft.

From the above it should be clear that exactly the same function andoperation of the fluid cylinder described can be obtained when hydraulicfluid is substituted for pneumatic fluid as the source of pressure.

A second embodiment of the present invention is disclosed in FIGS. 4 to6, wherein similar parts to those of the first embodiment are indicatedwith similar reference numerals. The general construction of this secondembodiment is the same as that of the first embodiment with thefollowing differences.

A pair of plungers 12,12 are reciprocably engaged respectively within apairof bores 9',9' drilled through the piston 4 parallel to the centralthrough bore 5. The left end of each of the plungers 12',12' is firmlyfixed to the pressure receiver plate 11 by means of the stop ring 13,while the right end thereof has a flange 12a. The through bores 9',9'comprise counterbores 9a and 9b, respectively, at the left and rightends thereof.

First springs 22a,22a' are disposed between the pressure receiver plate11 and the right end bases of the bores 9a,9a, and second springs22b',22b are interposed between the flanges 12a',12a" and the left endbases of the bores 9b,9b. The springs 22a,22a and 22b',22b' bias theplungers 12,l2', respectively, leftward and rightward to hold theplungers l2,12' in their neutral positions. A rotor 26 is rotatablydisposed in a radial bore 18' provided adjacent one of the plungersl2',12 within the piston 4. An elliptic cam 17 is firmly mounted at theouter end of the rotor 26 and a pinion gear 29 is in mesh with a rack 28formed along, or affixed to the plunger 12' adjacent the radial bore 18'as is best shown in FIG. 5.

Within the annular groove 16' on the outer circumferential wall of thepiston 4, is placed an annular brake band 15' made of resilient materialsuch as synthetic resin, or the like. A part of the brake band 15' iscut off so as to have radial cut-off faces 15a, 15b as shown in FIG. 6.

The second embodiment operates in the manner described below in detail.For the sake of convenience, pneumatic fluid is again used as thepressure source. In the drawings, FIGS. 4 to 6 show a state whereinpressure in both of the cylinder chambers 23a and 23b is equal toatmospheric pressure. Under this condition, the pressure receiver plate11 and the plungers 12, 12' are in their neutral positions, as shown inFIG. 4, due to the biasing force of the springs 22a',22a and 22b,22b'.The elliptic cam 17 of the rotor 26 is held at the position, shown inFIG. 6, in which the circumferential portions adjacent the longerdiameter of the cam contact the cut-off faces 15a and 15b of the brakeband 15. This expands the brake bands 15' so as to engage frictionallywith the inner circumferential wall of the casing l, and the pistonmeans 4 is firmly locked against the cylinder casing 1. In thiscondition, even if a load is given, added, or removed, to or from thepiston rod 2, the piston 4 is firmly kept in its locked position withinthe cylinder casing l by the locking effect of the brake band 15'.Therefore, the piston rod 2, as well as the piston 4, can be held attheir predetermined neutral positions without moving for a desiredperiod of time.

When pneumatic pressure is supplied to the cylinder chamber 23a, whilepressure in the chamber 23b remains equal to atmospheric pressure, thepneumatic pressure acts on the pressure receiver plate 11. This causesthe plate 11 and the plunger 12', 12' to move rightward against thebiasing force of the springs 22a',22a. The rightward movement of theplungers 12",12' in turn, produces rotation of the rotor 26. Inaccordance with the rotation of the rotor 26, the elliptic cam 17 isrotated through movements of the rack portion 28 of the plunger 12' andthe pinion gear 29. Thus, the circumferential portions of the camadjacent its longer diameter move away from the cut-off faces 15a and15b of the brake band 15', and the circumferential portions of the camadjacent its shorter diameter contact the cut-off faces 15a and 15b,releasing the locking condition of the brake band 15 against the innercircumferential wall of the casing 1. This permits the piston rod 2 aswell as the piston means 4 to move smoothly to the right.

When the pressure in both of the chambers 23a and 23b is equalized toatmospheric pressure the rightward movement of the piston 4 togetherwith its rod 2 is instantaneously stopped and the pressure receiverplate 11 as well as the plungers 12', 12' are returned to their neutralpositions, as shownin FIG. 4, by the biasing force of the spring22a',22a'. This causes the elliptic cam 17' to rotate so as to againcontact its circumferential portions adjacent the longer diameter firmlyagainst the cut-off faces 15a and 15b of the brake band 15' so as toexpand and lock the brake band 15 strongly to the tubular cylindercasing 1. Thus, the piston rod 2, as well as the piston 4, is againstably held at the desired position to the right without moving for anydesired period of time.

When pneumatic pressure from a source is directed into the chamber 23bwhile the pressure in the chamber 23a is equal to atmospheric pressure,the pneumatic pressure is lead into the chamber 25 through the pas"sageway 24, and pushes the pressure receiver plate 11 leftward againstthe biasing force of the springs 22b',22b'. The leftward movement of thepressure receiver plate 11 simultaneously causes leftward movement ofthe plungers 12', 12', the elliptic cam 17' being rotated through thepinion gear 29 meshed with the rack portion 28 of the plunger 12'. Therotation of the elliptic cam 17' releases the locking of the brake band15' against the inner circumferential wall of the casing 1 in the samemanner as discussed hereinabove. Thus, the piston means 4 and the pistonrod 2 can, again, move smoothly without interference from .the brakeband.

When the pressure in the chamber 23b is decreased to atmosphericpressure, the springs 22b,22b' return the plungers 12',ll2' to theirneutral positions to lock the brake band 15' against the inner wall ofthe casing 1. This locks the piston in a desired leftward position asdescribed above, so long as pneumatic pressure is not directed intoeither one of the cylinder chambers 23a and 23b, the elliptic cam 17'remaining turned so as to expand the brake band into frictionalengagement with the inner circumferential wall of the tubular cylindercasing 1. The piston means 4 and the piston rod 2 can, therefore, bestably held at the predetermined leftward position without moving forany desired period of time, regardless of the variations of the loadagainst the piston rod 2 and/or leakage of the operation fluid.

Upon delivery of pneumatic pressure into either one of the cylinderchambers 23a or 23b, the locking of the brake band 15 against thetubular cylinder casing 1 is instantaneously released, and the piston 4and the piston rod 2 are able to move very smoothly.

Although the use of pneumatic fluid has been adopted throughout thisdescription for the sake of convenience, it should be understood thatexactly the same effect, or mode of functioning can be obtained by useof hydraulic fluid instead of pneumatic fluid.

Although certain specific embodiments of the invention have been shownand described, it is obvious that many modifications thereof arepossible. The invention, therefore, is not intended to be restricted tothe exact showing of the drawings and description thereof, but isconsidered to include reasonable and obvious equivalents.

I claim:

l. A pressure fluid cylinder comprising a tubular casing, a piston witha connected piston rod reciprocable within said casing and defining twopressure chambers one on each side of the piston, brake means interposedbetween the outer wall of said piston and the inner wall of said casing,and means for operating said brake means mounted on said pistonincluding pressure receiving means operable to engage the brake meansagainst the casing when pressure in both of said pressure chambers isthe same and to release the brake means when the pressure in one of saidpressure chambers exceeds that in the other, said pressure receivingmeans including an annular member axially reciprocable within saidpiston to form a subpressure chamber,-

said subpressure chamber being connected by a fluid passageway to one ofsaid pressure chambers.

2. A pressure fluid cylinder according to claim 1, wherein said brakemeans comprises at least oneannular brake band disposed within anannular groove formed in the outer wall of said piston.

3. The pressure fluid cylinder as claimed in claim 2, wherein said brakeband is made of synthetic resin.

4. A pressure fluid cylinder comprising a tubular casing, a piston witha connected piston 'rod reciprocable within said casing and defining twopressure chambers one on each side of the piston, brake means interposedbetween the outer wall of said piston and the inner wall of said casing,and means for operating said brake means mounted on said pistonincluding pressure receiving means operable to engage the brake meansagainst the casing when pressure in both of said pressure chambers isthe same and to release the brake means when the pressure in one of saidpressure chambers exceeds that in the other, said brake means comprisingat least one annular brake band disposed within an annular groove formedin the outer wall of said piston, said annular brake band beingresilient and having 8. a radial self-returning force, said brake bandhaving a cut-off portion, and said operating means including meansengaging said cut-off portions for outwardly expanding said brake bandto frictionally engage the inner wall of said cylinder casing.

5. A pressure fluid cylinder according to claim 4, wherein saidoperating means includes a movable plunger coupled to said pressurereceiving means and having a neutral position in which said brake isexpanded and engaged against said casing, resilient means for normallyconditioning said plunger to its neutral position, movement of saidplunger from its neutral position causing said operating means to release said brake band.

6. The pressure fluid cylinder as claimed in claim 5, wherein said meansengaging said cut-off portion comprises a ball disposed in a radial boreprovided within said piston, said ball being outwardly thrust into saidcut-ofi portion to expand said brake band and being inwardly thrust outof said cut-off portion by the selfreturning force of said brake band.

7. The pressure fluid cylinder as claimed in claim 5, wherein saidoperating means includes an elliptical cam disposed within the saidcut-off portion, said cam being mounted on a rotor rotatable within aradial bore provided in said piston, a pinion gear on said rotor, and arack on said plunger in mesh with said pinion gear, said cam beingrotated to expand said brake band by movement of said plunger.

8. The pressure fluid cylinder according to claim 6, wherein saidcut-off portions of the brake band are outwardly tapered toward oneanother.

9. The pressure fluid cylinder as claimed in claim 7, wherein saidcut-off portion of the brake band has straight faces disposedapproximately radially of the piston.

10. The pressure fluid cylinder as claimed in claim 8, wherein saidplunger has a pair of annular grooves separated by a land, said plungerbeing axially reciprocable within said piston to thrust said balloutwardly into said cut-ofi portion bymeans of said land.

11. A pressure fluid cylinder as claimed in claim 10, wherein saidresilient means for normally conditioning said plunger to itsneutralposition comprises a pair of coiled springs disposed at the ends of asecond plunger reciprocable within said piston parallel to the axis ofthe piston, said first and second plungers being coupled by saidpressure receiving means.

12. A pressure fluid cylinder comprising a tubular casing, a piston witha connected piston rod reciprocable within said casing and defining twopressure chambers one on each side of the piston, brake means interposedbetween the outer wall of said piston and the inner wall of said casing,and means for operating said brake means mounted on said pistonincluding pressure receiving means operable to engage the brake meansagainst the casing when pressure in both of said pressure chambers isthe same and to release the brake means when the pressure in one of saidpressure chambers exceeds that in the other, said brake means comprisinga pair of annular brake bands disposed within an annular groove drilledalong the outer wall of said piston means, each of said brake bandshaving a cut-off portion of which the cut-off faces are taperedoutwardly toward one another, said cut-off portions being positioned ondiametrically opposite sides of the outer wall of said piston, and saidoperating means being operable to expand and release both of said brakebands.

' I II =0

1. A pressure fluid cylinder comprising a tubular casing, a piston witha connected piston rod reciprocable within said casing and defining twopressure chambers one on each side of the piston, brake means interposedbetween the outer wall of said piston and the inner wall of said casing,and means for operating said brake means mounted on said pistonincluding pressure receiving means operable to engage the brake meansagainst the casing when pressure in both of said pressure chambers isthe same and to release the brake means when the pressure in one of saidpressure chambers exceeds that in the other, said pressure receivingmeans including an annular member axially reciprocable within saidpiston to form a subpressure chamber, said subpressure chamber beingconnected by a fluid passageway to one of said pressure chambers.
 2. Apressure fluid cylinder according to claim 1, wherein said brake meanscomprises at least one annular brake band disposed within an annulargroove formed in the outer wall of said piston.
 3. The pressure fluidcylinder as claimed in claim 2, wherein said brake band is made ofsynthetic resin.
 4. A pressure fluid cylinder comprising a tubularcasing, a piston with a connected piston rod reciprocable within saidcasing and defining two pressure chambers one on each side of thepiston, brake means interposed between the outer wall of said piston andthe inner wall of said casing, and means for operating said brake meansmounted on said piston including pressure receiving means operable toengage the brake means against the casing when pressure in both of saidpressure chambers is the same and to release the brake means when thepressure in one of said pressure chambers exceeds that in the other,said brake means comprising at least one annular brake band disposedwithin an annular groove formed in the outer wall of said piston, saidannular brake band being resilient and having a radial self-returningforce, said brake band having a cut-off portion, and said operatingmeans including means engaging said cut-off portions for outwardlyexpanding said brake band to frictionally engage the inner wall of saidcylinder casing.
 5. A pressure fluid cylinder according to claim 4,wherein said operating means includes a movable plunger coupled to saidpressure receiving means and having a neutral position in which saidbrake is expanded and engaged against said casing, resilient means fornormally conditioning said plunger to its neutral position, movement ofsaid plunger from its neutral position causing said operating means torelease said brake band.
 6. The pressure fluid cylinder as claimed inclaim 5, wherein said means engaging said cut-off portion comprises aball disposed in a radial bore provided within said piston, said ballbeing outwardly thrust into said cut-off portion to expand said brakeband and being inwardly thrust out of said cut-off portion by theself-returning force of said brake band.
 7. The pressure fluid cylinderas claimed in claim 5, wherein said operating means includes anelliptical cam disposed within the said cut-off portion, said cam beingmounted on a rotor rotatable within a radial bore provided in saidpiston, a pinion gear on said rotor, and a rack on said plunger in meshwith said pinion gear, said cam being rotated to expand said brake bandby movement of said plunger.
 8. The pressure fluid cylinder according toclaim 6, wherein said cut-off portions of the brake band are outwardlytapered toward one another.
 9. The pressure fluid cylinder as claimed inclaim 7, wherein said cut-off portion of the brake band has straightfaces disposed approximately radially of the piston.
 10. The pressurefluid cylinder as claimed in claim 8, wherein said plunger has a pair ofannular groOves separated by a land, said plunger being axiallyreciprocable within said piston to thrust said ball outwardly into saidcut-off portion by means of said land.
 11. A pressure fluid cylinder asclaimed in claim 10, wherein said resilient means for normallyconditioning said plunger to its neutral position comprises a pair ofcoiled springs disposed at the ends of a second plunger reciprocablewithin said piston parallel to the axis of the piston, said first andsecond plungers being coupled by said pressure receiving means.
 12. Apressure fluid cylinder comprising a tubular casing, a piston with aconnected piston rod reciprocable within said casing and defining twopressure chambers one on each side of the piston, brake means interposedbetween the outer wall of said piston and the inner wall of said casing,and means for operating said brake means mounted on said pistonincluding pressure receiving means operable to engage the brake meansagainst the casing when pressure in both of said pressure chambers isthe same and to release the brake means when the pressure in one of saidpressure chambers exceeds that in the other, said brake means comprisinga pair of annular brake bands disposed within an annular groove drilledalong the outer wall of said piston means, each of said brake bandshaving a cut-off portion of which the cut-off faces are taperedoutwardly toward one another, said cut-off portions being positioned ondiametrically opposite sides of the outer wall of said piston, and saidoperating means being operable to expand and release both of said brakebands.